1. Field of the Invention
The present invention relates to an inverter power supply which drives a magnetron and as used for microwave ovens.
2. Description of the Prior Art
In an inverter power supply which drives a magnetron and fluorescent light, an A.C. input current waveform from a commercial A.C. power supply doesn't present a sinusoidal waveform congruent to an A.C. input voltage waveform, due to a nonlinear characteristic of voltage-current of a load and a rectifying-smoothing circuit of capacitor input type. In a familiar case, when an air condition is on with full power on a hot summer day and a microwave is on as well, other electric appliances at home, such as a television set sometimes do not operate as they should. This is because when the air condition and microwave oven consume high power, this can cause a television set to operate somehow not perfect, for example images on the television is not clear as they should be. In particular, due to the rectifying-smoothing circuit, a peak-type wave form is produced instead. As a result, a power factor of the A.C. input is deteriorated. To alleviate such a drawback, there has been conventionally considered an inverter power supply utilizing an active filter 32 in an input rectifying circuit as shown in FIG. 1. In the conventional apparatus, there are provided the first inverter 30 and the second inverter 40 encircled in dotted lines in FIG. 1. Precisely speaking, the second inverter 40 is indicated as encircled with the dotted line excluding a magnetron 39. Referring still to FIG. 1, A.C. voltage from a commercial AC power supply 31 is rectified by a rectifying bridge 33 in the inverter 30, then is switched periodically by a first switching element 34 to be converted to a high frequency. Then, an envelope of the switching current is controlled by a PWM (Pulse Width Modulation) control so that the envelope is congruent to the waveform of the A.C. input voltage, and an averaged value of the input voltage over the second inverter 40 is controlled at a constant level. The second inverter 40 is such that the input electric power to the magnetron 39 is controlled at a constant level according to the drive condition, by an electric power error integrator 36, a voltage/ON-period converter 37, a synchronous control circuit 38 and so on.
In connection with the aforementioned, FIG. 2 shows a conventional high-frequency power converter employing a method for controlling an anode current of the magnetron 39 at a constant level. In the high-frequency power converter, there is provided a control system for controlling an input power supplied to the magnetron 39 at a constant level. A current transformer 24 for detecting the averaged value of anode current is connected between a voltage doubler rectifier circuit 101 (not shown in FIG. 2) and an anode of the magnetron 39. Note that the voltage doubler rectifier circuit 101 can be seen in FIG. 5. A detection output of the current transformer 24 is converted to a D.C. voltage by a rectifier and is sent to an inversion input terminal (-) of an input power error integrator 36. Then, the power error from the input power error integrator 36 is controlled to be nil. As a result, the anode current of the magnetron 39 is controlled at a constant level and the input power is thus controlled at a constant value.
In the conventional inverter power supply, the first inverter and the second inverter are connected in series, so that its efficiency is a multiplied result of each efficiency, say, 90% times 90% making 81% which is a deterioration from the single efficiency of 90%. Further, there are more items of parts necessary and its physical size becomes bigger overall, thus causing complexity in construction and a cost increase. Moreover, since the switching frequency of the two inverters varies independently from each other, the operation frequency for each inverter must be differed a great deal, namely at least by 20 kHz, in order to have the difference in frequency set outside the audible frequency range. As a result, a switching loss is remarkable, especially in the inverter having a greater operation frequency.